1. Field of the Invention
The present invention relates to an image forming apparatus such as a multi functional peripheral or a printer.
2. Description of the Related Art
One type of known image forming apparatus is a multi functional peripheral that includes a photosensitive drum with surfaces on which an electrostatic latent image is formed, a developing unit that converts the electrostatic latent image into a toner image, and a transfer roller that directly or indirectly transfers the toner image onto a recording medium. In such a multi functional peripheral, a transfer bias (voltage) is applied to the transfer roller in order to directly or indirectly transfer the toner images formed on the photosensitive drums onto a recording medium. One known image forming apparatus applies a transfer bias to the transfer roller via a spring material by a structure in which the spring material contacts a bearing supporting a roller shaft of the transfer roller from the side of the outer circumferential surfaces of the bearing.
Furthermore, another known color multi functional peripheral uses an intermediate transfer belt and a secondary transfer roller. With such a color multi functional peripheral, a full color image is formed on the surface of the recording medium by transferring a full-color toner image, which is formed by overlapping different color toner images, onto the sheet by using the intermediate transfer belt and the secondary transfer roller.
In such a color multi functional peripheral, the secondary transfer roller is moved by a transfer-roller moving unit between a contact position where the secondary transfer roller contacts the intermediate transfer belt with the sheet interposed therebetween and a spacing position where the secondary transfer roller is spaced apart from the intermediate transfer belt.
When the secondary transfer roller is in the contact position, the secondary transfer roller to which a secondary transfer bias is applied is disposed such that the sheet is sandwiched between the secondary transfer roller and the intermediate transfer belt. Thus, the sheet is pressed against the intermediate transfer belt, and the full-color toner image primarily transferred onto the intermediate transfer belt is secondarily transferred onto the surface of the recording medium.
There is a known color multi functional peripheral that applies a secondary transfer bias via spring material by employing a structure in which the spring material contacts the outer circumferential surface of the rotary shaft of a secondary transfer roller, and the spring material follows the movement of the secondary transfer roller. In such a color multi functional peripheral, even when the secondary transfer roller is moved by the above-mentioned transfer-roller moving unit, since the spring material follows the movement of the secondary transfer roller, a secondary bias can be applied to the secondary transfer roller via the spring material.
With a color multi functional peripheral having such a structure, when the secondary transfer roller is in the contact position, the spring material necessarily follows the movement of the secondary transfer roller and contacts the outer circumferential surface of the rotary shaft in a stable condition. Therefore, the structure of the color multi functional peripheral becomes complicated. Another problem is that the contact state of the spring material and the secondary transfer roller may become unstable due to deterioration of the resilient force of the spring material.
When the peripheral has the above-mentioned transfer-roller moving unit to move the secondary transfer roller, such problems may occur not only in color multi functional peripherals that apply a secondary transfer bias to a secondary transfer roller but also in image forming apparatuses that apply a voltage as a transfer bias to transfer rollers such as a primary transfer roller and a direct transfer roller.